A visual field test, also known as perimetry, is a method of measuring an individual's entire scope of vision, that is, the central and peripheral vision. Such visual field tests attempt to map the visual field of each eye individually. Visual field testing is most frequently used to detect any signs of glaucoma-caused damage to the optic nerve. In addition it is useful for detection of central or peripheral retinal disease, eyelid conditions such as ptosis or drooping, optic nerve disease, and diseases affecting the visual pathways within the brain and associated with the Central Nervous System (“CNS”).
The present prevailing method for visual field testing is performed as follows: one eye of the patient is covered, and the chin is placed on a concave chin rest. The patient must look straight ahead at all times in order to avoid testing the central vision rather than the periphery. Next, light flashes of various intensities are projected on the tested eye or onto a testing bowl at different locations. Whenever the patient notices a flash, he or she is instructed to push a button. After all the relevant viewing angles are covered, a computer program analyzes the patient's responses and assesses the visual field map of the tested eye.
The principal stumbling block of the above procedure is its subjectivity, requiring the patient to understand the testing instructions, fully cooperate, and complete the entire test in order to provide useful information. However, the patient cooperation may strongly depend on his or her level of fatigue, wakefulness and attentiveness. This problem is especially severe in case of ill or elderly patients, younger children or patients with mental disabilities and developmental delay. Consequently, the test results obtained by the current method may not be accurate and may lead to false medical diagnosis. Moreover, the results may not be repeatable, which does not allow for reliable and effective tracking of the patient's medical condition.
Additional tests to assess the state of the eye include the Pupillary Light Reflex (“PLR”) to provide clinical signs of the condition associated with the CNS. The PLR tests the pupil response, namely constriction, by testing the pupil's response to light stimuli in each eye, where a healthy eye may be indicative of symmetric constriction of both pupils. A quantitative measurement of a PLR may be obtained using a pupillometer.
Pupil perimetry utilizes a pupillometer together with a stimulus arrangement similar to that of a perimeter to measure the latency and amplitude of the constriction of the pupil in response to stimuli, usually in the form of a spot (“small-area”) of flashes of light that are directed to different locations on the retina.
The pupillary response to spatially-localized luminance increments has been used as an indicator of glaucomatous retinal damage, but the small-area stimuli used in pupil perimetry may target small retinal areas that only weakly stimulate a PLR, and may fail to stimulate a PLR if the small retinal area that is being stimulated by light has been damaged by glaucoma.
Standard pupil perimetry testing produces large variations in pupil response amplitude among patients, and the changes in sensitivity of the pupil response with the retinal location of the small-area light stimulus have also limited the usefulness of such measurements.
Pupillometer-based objective visual tests have been recited in some references, based on an achromatic beam stimulus which is applied at various angles, for example, U.S. Pat. No. 5,610,673 to Rafal et al, U.S. Pat. No. 7,524,064 to Wyatt, U.S. Pat. No. 7,258,444 to Gorin. However, these methods fail to achieve either accurate or repeatable visual field mapping due to their susceptibility to time variations in the human ocular system and to differences in the behavior of the ocular system of different patients.